Various particle sizing samplers are available on the market today for sampling, according to size, particulate pollutants entrained in a gaseous medium such as air or stack gases. These samplers are generally of two different types, the jet impaction type sampler and the cyclone type sampler. The jet impaction type sampler serially directs jets of the gaseous medium against impaction surfaces at different velocities in ascending sequence so that different size particles are separated at the different velocities. The cyclone type sampler uses a series of cyclones through which the gaseous medium serially passes with each cyclone separating different size particles. Both of these types of sampler suffer from certain problems.
One of the problems associated with prior art jet impaction type particle sizing samplers is that such samplers generally have a capability of collecting and sizing only small quantities of particles from the gaseous medium without the particles which already have been collected by jet impaction being reentrained in the gaseous medium. When this occurs, of course, the sizing accuracy of the sampler deteriorates, thereby making it difficult to determine what size particles were collected from the gaseous medium. Because the particle concentrations in gas streams which are being sampled vary widely, the jet impaction type sampler is frequently unable to take a sample of the gaseous medium for the necessary length of time to conduct proper sampling in gas streams with high particle concentrations.
One of the problems associated with cyclone type samplers is that, at any given volumetric flow rate, a particular size cyclone has to be associated with each particle size to be collected since the size of the cyclone itself determines the size of the particles which are collected by that cyclone. As a result, the physical arrangement of the cyclones in the sampler is difficult to achieve while at the same time keeping the overall sampler size small enough to fit through the standard sampling ports which are normally available in ducts carrying the gaseous medium. An even greater problem with cyclones is that there is no satisfactory theoretical formulation from which to predict the particle size which a cyclone will collect at various combinations of pressure and temperature. Eacy cyclone must be calibrated empirically at the possible combinations of gas temperature and gas pressure which might be encountered in actual field use. This calibration procedure is expensive and subject to errors. Another problem which has been encountered with cyclone type samplers is that some of the particles may be removed from the gaseous medium within the complex passage arrangements required to interconnect the different cyclones rather than at the desired collection points in the cyclones. This causes inaccurate results in the indicated particle size distribution in the gaseous medium. Yet another problem encountered with the cyclone type sampler is that some of the separated particles may be reentrained in the gaseous medium to thereby reduce the collection size accuracy of the sampler.